Blender game engine seeing through gun
Normals are pointing inwards 

Solution: flip them.

But: They do look right at your picture.

Transparency enabled. 

Solution: disable transparency in the material tab. As your model does not need any transparency this should be no issue. 

Mirrored the object. That makes the normals point towards the opposite direction (see the first option). This can happen on negative scale on one or three axis (scale by -1). It is possible you have an according modifier mirroring your mesh.

Solution: Mirror again, scale back, flip normals and/or review your modifiers.